UK feeds I'm aware of, in case they're helpful for anyone:

Great Britain intercity rail

RDG (aka "National Rail", aka ATOC, the consortium of train operators) publishes timetable data in their own "CIF" format, which covers all intercity rail in Great Britain (this includes Merseyrail, Liverpool's metro system, and S-Bahn-style services in several cities including London, but not the London Underground, Glasgow Subway, or any of the tram/light rail systems in the country). Also includes some bus and ferry timetables where through tickets exist, but not comprehensive.

Documentation on accessing this (requires an account, but otherwise simple enough) is here, and docs are here. I'm pretty sure there are GTFS converters available on GitHub, but this would lose some data that's not supported by GTFS (such as minimum transfer times at stations, and possible walking/metro connections between stations).

Great Britain Busses

Available in GTFS from the Bus Open Data Service.

Traveline also claims to have a nationwide dataset of busses, trams, and light rail, but they didn't respond to my request for an account a few months ago.

Eurostar

Available in GTFS from the French government transport data portal.

Northern Ireland

Rail data apparently available in CIF here, though I haven't investigated in any detail.

Bus data is available here, apparently in a version of CIF modified for bus usage (their own peculiar innovation, I think) and split between Belfast and everywhere else.